1. Field of the Invention
The present invention relates to an image processing apparatus in an electronic file, a facsimile or a digital copying machine, and more particularly to an image processing apparatus having a function to discriminate an image content.
2. Description of the Prior Art
In an image processing office equipment such as a facsimile or a copying machine, there are two inconsistent demands, one being sharp reproduction of a boundary of black and white areas of an image such as a character or a graphic image and the other being gray level reproduction with a vignette boundary of black and white areas of an image such as a photographic image.
The former of these two inconsistent demands has been satisfied by increasing a resolution power of the image of the apparatus and the latter has been substantially satisfied by a dither method recently developed as digital technology advanced.
However, an original image which a user inputs to the office equipment is not always limited to a binary image such as characters or graphic patterns or a gray level image such as photograph but in many cases it includes a mixture of the binary image and the gray level image. Since it is very difficult to separate the binary image area and the gray level image area of the original text which includes the mixture of the binary image and the gray level image, the original text has been processed either by a binary recording technique (in which an image data is binarized by a predetermined threshold and recorded by ON dots and OFF dots) or a gray level recording technique (in which the image data is binarized by a dither matrix and recorded by ON dots and OFF dots). However, the binary recording has the advantages that it can sharply reproduce the characters and graphic pattern and readily enable band compression encoding of a transmission signal but it has a disadvantage in that it is difficult to reproduce a gray level image such as a photograph with a high quality. On the other hand, the dither method can reproduce a gray level of a gray level image but has a poor resolution power and hence the quality of image is low for the binary images such as characters. Accordingly, when the original image including a mixture of gray level images and binary images is processed by one or the other of gray level recording and binary recording, only one of these types of images can be reproduced with high quality.
In order to resolve the above problem, it has been proposed to use the systematic dither method in the gray level image area and the binary processing method in the binary image area such as characters or pattern to reproduce a high quality of image.
In this case, however, an image memory which has a one-to-one correspondence to pixels read from the original image is necessary in order to discriminate the binary image area and the gray level image area. Accordingly, depending on the size of the image to be processed, the pixel density and the number of gray levels to be processed, the memory capacity increases and a complex circuit is required. As a result, the cost of the memory increases, the reliability of the apparatus is reduced and the operation of the memory is complicated.